Crack appearance

Although the majority of the hydrogen produced on the cathodic areas is evolved as gas and assists the removal of scale, some of it diffuses into the steel in the atomic form and can render it brittle. With hardened or high-carbon steels the brittleness may be so pronounced that cracks appear during pickling. Austenitic steels, however, are not so subject to embrittlement. 

Thermal cracking appears to be similar, but in this case air is the aggressive environment, becoming active at 70-80 C with some polyethylenes. 

Biscuit Mixes. Biscuit mixes are often packaged in printed paper/ heat sealant-coated pouches. At one time the paper was coated with polyethylene and PVDC (as the sealing medium) for grease resistance, since many biscuit mixes contain up to 12% shortening. Many grease spots appeared on pouches because of cracks appearing in the brittle PVDC... 

Caustic embrittlement corrosion is intergranular, and cracks appear along the grain boundaries. The process is accelerated by ... 

Polythene bottles are not suitable for long-term storage of plutonium nitrate solutions as radiation-induced stress cracks appeared in the bases of several 10 1 bottles during 6 months storage. Short-term storage and improved venting are recommended. 

Figure 7.3 Cracks appear in the coating of optical fibres if the organic polymer content is reduced below a certain threshold. (Reproduced from ref. 4, with permission.)...

Timely and consistent maintenance provides assurance that the system is physically in the condition intended. When more than hairline cracking appears, the openings should be cleaned out and filled with new material according to the manufacturer s instructions. Loss of bonding to the substrate may be detected by surface bulges or an abnormal sound when the surface is tapped with a light hammer. 

During a normal development process, the cracks appear only in the immediate vicinity of the exposed area and almost invariably originate from the corners of patterns as shown in Figure 3. 

Static joints are the subject of a special form of Murphy s Law Pinholes and cracks appear most frequently in the least accessible places. The reason is that just because the place is accessible only with difficulty, the working of the glass during the making of the joint will have been hampered and therefore probably not well done. That is why the experienced leak hunter will first suspect a leak at the backs of joints made close to any fixtures, i.e. the parts of the joints which are difficult to see, to get the flame to, and to inspect. 

Palmquist (1957) was the first to make use of the deformation developing around the Vickers pyramid indentation as an aid in the interpretation of results. The Palmquist test consists of determining the resistance of brittle materials to propagation of the cracks appearing at the corners of the Vickers pyramid mapping on a polished surface. The measured value, defined as Palmquist toughness, is given by the formula... 

Cracks appear when the nitrourea is filtered hence, care must be taken that the water used for washing does not go through the cracks instead of through the material. If desired, the nitrourea may be removed from the funnel, made into a paste with about 800 cc. of cold water, and again filtered. 

As a result of the increase in stress and/or strain, shear bands develop in a large fraction of the sample but, at a certain point, a crack appears and starts to propagate. Several mechanisms for energy absorption, associated with the presence of particles, become active during crack propagation. 

The problem of osmotic cracking is well known for polyester boat and swimming pool users. After a more or less long time of exposure in water, blisters and cracks appear at the surface. Their formation can be detected sometimes gravimetrically (Fig. 14.10). It can be shown that the induction time of osmotic cracking, tb, is (Gautier et al., 1999) ... 

The morphological parameter C which controls the damaging effect of cracks appears to depend only on the crack density and the crack radius. In the case of cracks, the macroscopic mechanical loading defined by Ii or p is expected to induce a strong heterogeneity of the mechanical affinity, due to the stress/strain concentration in the vicinity of the crack tips. 

Observations of pure MgO quenched into oil from <600°C, on polished surfaces and cross-sections through the centres of the bars, show no cracks however, at the critical quench temperature (ATC), 600°C, a large number of cracks appear on the polished surfaces, and a small number progress towards the centres of the bars (Fig. 14.14(a)). The amount of cracks increases markedly at quenching from 1000°C, and cracks are now visible on both polished surfaces and cross-sections through to the centres of the bars most cracks penetrate more than one-quarter of the distance through the bar. 

With optimum selection of fibres and matrices, favourable residual stress conditions can be established in the matrix, which lead to increased A Tc. Above A Tc, matrix cracks appear but the presence of crack-deflecting fibre-matrix interfaces ensures minimal effect on mechanical properties as the fibres remain largely unaffected. As damage is also confined mostly to the surface of the materials, changes in mechanical and thermal properties are more readily identified by means other than mechanical testing. 

Matrix cracks that originated from the corners of uninfiltrated pores in regions outside fibre bundles. These cracks appeared at AT= 250°C and did not penetrate deeply into the fibre bundles, though the penetration depth increased with increasing A T. 

Fig. 12.78. When a stress crack appears at the surface, this becomes a locus of an increased dissolution activity, the electrons being drawn away to the rest of the surface as the electronation area.

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